Electrolytic surface treatment of steel



Patented Feb. 27, 1951 ELECTROLYTIC SURFACE TREATMENT OF STEEL Harold W.Cobb, Baltimore, Md., assignor, by mesne assignments, to Armco SteelCorporation, a corporation of Ohio No Drawing. Application July 9, 1945,Serial No. 604,090

Claims.

This invention relates to metal articles and products, more especiallyto an art of producing black coated stainless steel products, and to theresulting treated products themselves, repreresenting an improvement tothe process and product of the pending application of Irvine C. Clingan,Serial No. 470,853, filed December 31, 1942, and entitled StainlessSteel and Method, now Patent 2,394,899 of February 12, 1946.

An object of my invention is the rapid, efficient and economicalproduction of black oxide coated stainless steel articles and products,for example, machine and equipment parts, accessories, ornamentalobjects and parts, utensils, implements, instruments and instrumentparts, trim and furnishings, or the like, by which the coating producedis resistant to corrosion and mechanical wear.

Another object is the production of coated stainless steel of thecharacter indicated in a simple, practical and thoroughly reliablemanner including the utilization of readily available materials andknown and tried equipment.

A further object of my invention is the provision of stainless steelproducts and articles having a high quality black oxide coating which isresistant to corrosive attack and wear such as by abrasion.

Other objects in part will be obvious and in part will be pointed outhereinafter.

The invention accordingly consists in the combination of elements,composition of materials, and conditions of operation, in the varioussteps, and in the relation of each of the same to one or more of theothers as described herein, the scope of the application of which isindicated in the following claims.

As conducive to.a clearer understanding of certain features of myinvention, it may b noted at this point that the stainless steels bydefinition comprise carbon ranging from negligible amounts up to 0.25%in the relatively low-carbon varieties and more for the higher-carbongrades, to 35% or more chromium, in many instances nickel, andfrequently supplemental additions of manganese, silicon, cobalt, copper,molybdenum, tungsten, vanadium, columbium, titanium, sulphur, and thelike, for special purposes, and the remainder substantially all iron.

The stainless steels most frequently are put into use having a naturalmetallic surface finish, which in the absence of polishing. is gray andsomewhat dull, or which with polishing as by means of rubbing, bufllngor electrolytic polish- 'ing treatment is highly reflective and bright.In

natural metallic finish, the stainless steels far surpass ordinarycarbon steels in corrosion resistance, the extent depending upon suchfactors as composition, treatment prior to use, and corrosiveenvironment. Even so, it is at times desirable to provide other thannatural metallic finishes on stainless steel products for such reasonsas to gain further resistance to corrosion, a light absorbing finish, ora departure from natural metallic appearance. As a result of existingneeds, a number of stainless steel treating processes which have fortheir object the provision of protective coatings or films on the metalsurfaces now form a part of the prior :art.

Many of the heretofore known coating proc-- esses, however, fall shortof acceptance for providing desired types of coatings on stainlesssteel. In particular, some of the processes give coatings which areremoved too readily in use, especially by rubbing, or by abrasion suchas with sand, grit, coarse cloth, leather or other abrasive substances.The removal by rubbing or by abrading often is more pronounced when thecoated products are additionally exposed to a wetting agent as forexample oil or water instead of being utilized in dry surroundings. Afurther prior art treatment of stainless steel such as painting givescoatings which are so thick as to be objectionable in producing coatedproducts within close dimensional tolerances.

There are certain known processes which include the production ofstainless steel coated with chemical reaction compounds of the steelitself.

A number of these processes, however, require an extensive time ofcoating treatment of the steel, evolve dangerous or obnoxious fumes, areextremely diflicult to control for consistent effectiveness, or entailother matters which are objectionable such as speedy deterioration ofthe treating chemicals and too frequent need for re-- placement of spentbath constituents. Some of the chemical processes do not yield desiredcorrosion resistant coatings or ones which are adherent to the stainlesssteels, while other known processes are ineflicient in view of inertnessof the metal to certain types of treating chemicals and accompanyingconditions of coating treatment.

Quite some few of, the chemically coated stainless steels are notresistant to abrasion or rubbing, instead are soft and rub off or wearaway too readily for most purposes. This is true of the heretofore knownhydroxide coatings, which in addition are aifected by moisture to suchextent as to undergo a change in appearance.

An outsanding object of my invention, accordingly, is the provision of amethod for producing black oxide coated stainless steel products, orother alloy steel products, and the products themserves, which coatingis hard, adherent, and resistant to abrasion in wet or dry environments,which is corrosion resistant, dense, and of uniform texture andappearance, and which, being hard and substantially free of chalkiness,does not rub off or wear away during extensive use.

Referring now more particularly to the practice of my invention, Iobtain hard-surface blackened ferrous metal, for example, stainlesssteel, by treatment which includes both immersing the same in a strongblackening salt bath of sodium dichromate and/or potassium dichromate,and then subjecting the metal in blackened condition to electricalcathodizing current, preferably in a separate dichromate bath. Thequality of black coating which I achieve, including physical depth ofthe coating and hardness and abrasion resistance of the same, depends toa large extent upon such factors as time of immersion, temperature ofthe immersion bath, and conditions of the oathodizing treatment.Likewise, the actual appearance of the coating importantly reverts tosuch controls as initial finish of the article or product beforetreatment. A dull gray unpolished piece responds to my blackening andcathodizing treatment by assuming a resulting dull black appearance,while a polished piece assumes a smooth black finish as a result of thetreatment.

As illustrative of the practice of my invention, I provide stainlesssteel articles and products for black coating treatment, for example,those in the form of telephone equipment parts, door and window trim,light switch plates, gun parts, vacuum cleaner or electric toastercasings, handles and bumpers, photographic and optical equipment parts,or in the formof any of a wide variety of other stainless steel productsand articles to serve such functions after blackening as entail rubbing,abrasion, corrosive attack, or some further need for endurance of blackcoating. Before continuing with my coating treatment, I prefer topreliminarily cleanse the stainless steel articles or products, such asby pickling, to eliminate substantially all scale, oxide film, grease,dirt, or the like, often present on the metal surfaces as a result ofearlier treatment or fabrication. At times, I resort to mechanicalcleaning methods such as sand blasting or grinding with or without thepickling treatment.

For the pickling of stainless steel products, I find that dipping in anaqueous solution of 20% nitric acid and 1% hydrofluoric acid usually issufiicient. Thereafter, I subject the products to rinsing in cleanwater, finally obtaining a scalefree and otherwise clean metal surfacewhich usually is of dull gray or satin-like appearance. I subsequentlydry the products which then are ready for coating especially where adull black finish is sought.

In preparing the products for a smooth black finish, I usually introducea polishing step before the blackening operation. Mechanical polishingmethods such as rubbing or buffing often are satisfactory for thispurpose. An even better polishing step which I frequently employinvolves electrolytic polishing treatment of the metal, as for example,polishing in accordance with the process described and claimed in U. S.Patent No. 2,335,354 of James N. Ostrofsky, entitled Polishing StainlessIron and Steel, in which the stainless steel 4 products are made theanode of an aqueous electrolyte comprising an aliphatic carboxylic acidsuch as citric acid and a soluble compound having a sulphate radicalsuch as sulphuric acid, or

subject the products to alternating current treat- For the coating ofstainless steel articles and products, or other alloy products, inaccordance with my invention, I prepare in a melting vat or the likehaving a suitable heater, a molten salt bath consisting of sodiumdichromate and/or potassium dichromate. The preferred dichromate bathwhich I form and utilize for blackening the metal consists substantiallyentirely of fused sodium dichromate and/or potassium dichromate. Thedichromate salts are heated to temperatures which are sufficient to givea molten bath. In general, I maintain the blackening bath usually withinthe approximate temperature range of 320 C. to 500 C.

To effect a blackening treatment in accordance with my invention, Iimmerse a stanless steel product, for example, in the dichromate bathfor a period of time ranging from about 2 to 30 minutes or more. Thebath temperature preferably is substantially constant throughout theblackening period. In immersing the metal in the fused dchromate, Iresort to such practices as holding the metal immersed during the entireblackening treatment or to dipping the product repeatedly into the bath.The blackening treatment results in the achievement of a dense, adherentblack coating on the metal which in the instance of the sta'nless steelproduct consists of chromic oxide (CrzOa) as determined by X-rayanalysis.

Where the metal, such as the stainless steel product is blackened bydichromate bath treatment, I effect a further step which materially andimportantly contributes to the achievement of a hard, abrasion-resistantblack finish, as on the stainless steel product surface. In carrying outthis step, I subject the metal to direct current cathodizing treatmentwithin a fused dichromate bath after the blackening operation proper.Following the cathodizing treatment, the product is removed from thebath, rinsed and dried. Experience indicates that the abrasionres'stance of the resulting coating thus obtained usua.ly is superior ascompared with results of blackening the metal without cathodicallytreating the blackened product.

In cathodizing, I conveniently provide the treating current from asuitable source of direct current electrical supply to an anodeillustratively of stainless steel, for examp'e 18-8 chromium-nickel or17% chromium steel, in one of the dichromate baths describedhereinbefore, heated to within temperature limits as already defined,the work itself being made the cathode.

To cathodize and enhance surface hardness of the stainless steel productreferred to, for example, as after the product has been subjected for atleast about 2 minutes time to dichromate bath blackening treatment, Isupply direct current through the dichromate usually for 5 to 10 minutesor more to the product as cathode. In so doing, I maintain a currentdensity of about 0.05 ampere per square inch to approximately 4 amperesper square inch with respect toarea of product surface under cathodizingtreatment. A preferred cathodizing current density range which I use inthis connection extends from about 0.05 ampere per square inch up toabout 0.25 ampere per square inch. The optimum current density formaximum abrasion resistance of the resulting products is about 0.10ampere per square inch, but this, of course, may vary somewhat with thegrades of blackened metal which I cathodize, or it may be influenced bythe condition of the bath. With the stainless steel product blackcoated, and the cathodizing treatment complete, I withdraw the same andthereafter rinse and dry the coated metal surface.

The black coated and cathodized stainless steel product possesses adurable and beautiful black oxide finish which in no manner impairs theruthless or corrosion-resisting properties of the underlying metal. Myblack oxide film or coating itself is adherent, hard, and resistant toabrasion, rubbing or wiping whether exposed to wet or dry environments,and is dense and resistant to corrosion. The black oxide coating doesnot fade in atmospheres of varying humidity or even after extensiverubbing or abrasion, and maintains its natural appearance for a longperiod of time in the outdoor elements.

By actual test on a Taber abraser, under dry conditions, using astandard abrading wheel (No. CS-17F loaded at 1000 gms. and running at60 R. P. M. comparative results were had of my black coated andcathodized products, as compared to a product black coated but notcathodized, as follows:

Rev. to

Cathodiz Current Failure Sample No.

none.

.05 ampJsq. in .10 amp./sq. in.. .25 ampJsq. in......

Rev. to

Sample No. Failure Cathodizing Current none.

.05 amp/sq. in .10 amp/sq. in. .25 ampJsq. in..

In providing black coated products, and in improving the same bycathodizing, I find it advantageous to employ a molten salt bathcomprising substantially all sodium dichromate at temperatures of bathranging between 320 C. and 400 C., the melting and decompositiontemperatures, respectively, of sodium dichromate. The bath isparticularly useful for providing high quality cathodized coatings onchromium-nickel stainless steel products such as the 18-8 chromiumnickelgrades, but also is capable of contributing to the achievement ofabrasion-resistant, cathodized black oxide finishes on straight chromiumstainless steel products as well as on other alloy steel products. Ifind that the oxidizing strength of the bath increases with temperatureincreases. By holding the bath in a temperature range well above thedecomposition temperature, however, satisfactory black coatings onstainless steel are no longer rapidly achieved, although good coatingsare achieved by extending the treatingtime. In treating with the sodiumdichromate baths of the character just mentioned,

- I therefore prefer a process range of about 360 C. to approximately380 C. In the blackening treatment a highly satisfactory black oxidecoating or film develops on the products in about 2 to 15 minutes time.The same temperatures are suitable for the cathodizing treatment which,for example, I effect immediately following the blackening treatmentproper.

For coating and/or cathodizing stainless steel products such as those ofchromium or chromium-nickel steel, at bath temperatures: higher thanthose which are practical in using sodium dichromate alone, a moltenbath containing substantial amounts of sodium dichromate and potassiumdichromate is advantageous. The sodium dichromate of the bath is a moreactive oxidizing agent for stainless steel than is potassium dichromate,and, therefore, preferably is included in predominating amount,especially for effecting the blackening treatment in combination withthe cathodizing operation or followed by the same. In treating theproducts, temperatures of approximately 400 C. to 500 C. are employedwith the mixed salt bath.

Thus,it will be seen that there are provided in this invention an artand product in which the various objects hereinbefore noted, togetherwith many thoroughly practical advantages are successfully achieved. Itwill be seen that the product is made with assistance of cathodizingtreatment to have a durable, abrasion-resistant and corrosion-resistantblack oxide coating in a simple and thoroughly reliable manner withoutin any way impairing the rustlessor corrosion resistant characteristicsof the metal. Moreover, it will be seen that my process is readilypracticed with stable and highly effective chemicals which are easy toobtain and to handle.

While my process is described as being particularly useful in theprovision of cathodized black oxide finishes on stainless steel articlesand products, it will be understood that the coatings achieved may serveas a base for subsequent coatings or films such as paint, enamel, andthe like.

As many possible embodiments may be made of my invention and as manychanges may be made in the embodiment hereinbefore set forth, it 'willbe understood that all matter described herein is to be interpreted asillustrative and not in a limiting sense.

I claim as my invention:

1. In treating alloy steel articles containing at least 10% chromium toobtain a hard, adherent, abrasion-resisting surface, the art whichcomprises, surface-blackening such articles in a fused salt bathconsisting of salt of the group consisting of sodium dichromate andpotassium dichromate, and then subjecting the same to electricalcathodizing treatment for at least about five minutes in a fused saltbath consisting essentially of salt of the group consisting of sodiumdichromate and potassium dichromate.

2. In treating stainless steel articles to obtain a hard, adherent,abrasion-resisting surface, the art which comprises, surface-blackeningsuch articles in a fused bath consisting of salt of the group consistingof sodium dichromate and potassium dichromate heated to a temperature of320 C. to 500 C., and subjecting the blackened articles to electrolyticcathodizing treatment for about five to ten minutes in said fused saltbath at a current density of 0.05 to 0.25 ampere per square inch ofarticle area immersed.

3. In treating stainless steel articles to obtain a hard, adherent,abrasion-resisting surface, the art which comprises, surface-blackeningsuch articles in a fused salt bath comprising substa tially all sodiumdichromate heated to temper ture within the range of 360 C. and 380 C.,and then subjecting the same to electrolytic cathodizing treatment forabout five to ten minutes in a fused salt bath at a current density ofabout 0.10 ampere per square inch of article area immersed.

4. In treating alloy steel articles containing at least 10% chromium toobtain a hard, adherent, abrasion-resisting surface, the art whichcomprises electrolytically polishing such articles, then surfaceblackening the polished articles in a fused salt bath containingsubstantiaiiy all salt of the Group consisting of sodium dichromate andpotassium dichromate at a temperature of 320 C. to 500 C. for a periodranging from about 2 to 30 minutes time, and after at least said 2minutes blackening time subjecting the articles to electrolyticcathodizing treatment in said fused salt bath at a current density of0.05 to 4 amperes per square inch of article area immersed for a periodof at least 5 minutes.

5. An alloy steel product containing at least fused salt bath consistingof salt of the group consisting of sodium dichromate and potassiumdichromate, and subjected to electrical cathodizing treatment in a fusedsalt bath essentially consisting of salt of the group consisting ofsodium dichromate and potassium dichromate.

HAROLD W. COBB REFERENCES CITED The following references are of recordin the file of this patent:.

UNITED STATES PATENTS Number Name Date 1,795,512 Schmidt et al Mar. 10,1931 1,845,978 Hosenfeld Feb. 16, 1932 1,927,773 Chittum Sept. 19, 19331,975,239 Ungelenk Oct. 2, 1934 2,261,744 Ostrofsky Nov. 4, 19412,394,899 Clingan Feb. 12, 1946 2,426,962 Witchger Sept. 2, 1947 FOREIGNPATENTS Number Country Date 695,366 France of 1930 OTHER REFERENCESHackh's Chemical Dictionary, 3d edition (1944), pages 198, 199.

1. IN TREATING ALLOY STEEL ARTICLES CONTAINING AT LEAST 10% CHROMIUM TOOBTAIN A HARD, ADHERENT, ABRASION-RESISTING SURFACE, THE ART WHICHCOMPRISES, SURFACE-BLACKENING SUCH ARTICLES IN A FUSED SALT BATHCONSISTING OF SALT OF THE GROUP CONSISTING OF SODIUM DICHROMATE ANDPOTASSIUM DICHROMATE, AND THEN SUBJECTING THE SAME TO ELECTRICALCATHODIZING TREATMENT FOR AT LEAST ABOUT FIVE MINUTES IN A FUSED SALTBATH CONSISTING ESSENTIALLY OF SALT OF THE GROUP CONSISTING OF SODIUMDICHROMATE AND POTASSIUM DICHROMATE.